For a conventional electrophotographic copying method, a Carlson system wherein after uniformly electrostatically charging the surface of an electrophotographic photoreceptor, an imagewise exposure is applied to the surface to imagewise decay the surface potential, then the surface is developed by a toner, and the toner image formed is transferred onto a plain paper has been widely used as a general method. However, in the system, each one copying of an image requires each image exposure, whereby there is a problem that for high-speed copying, the apparatus is complicated and large-sized.
For solving the problem, a system of obtaining a large number of copies with one image exposure by storing an image information in an electrophotographic photoreceptor has been proposed. For example, JP-A-53-102037 and JP-A-2-269357 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describe that a durable photoreceptor is obtained by incorporating a specific compound in the photosensitive layer. However, by the aforesaid method, a satisfactory photoreceptor is hard to obtain in the points of the storable time of image information, the contrast at copying, etc.
On the other hand, U.S. Pat. No. 4,883,731 describes an electrophotographic system of obtaining a large number of copies by migrating photosensitive particles in the inside of a heat softening layer to store an image information.
FIG. 9 to FIG. 16 of the accompanied drawings are schematic views explaining the foregoing case. In these figures, a photosensitive master plate 80 is composed of a substrate 81, an electrically conductive layer 82, a heat softening layer 83 containing a heat-softenable thermoplastic resin and a charge transporting material, and photosensitive particles 84 embedded in the heat softening layer near the surface thereof and generating charges by light exposure.
At the use of the photosensitive master plate, as shown in FIG. 9, the surface thereof is first uniformly charged negatively by means of a corona charging device 60. Then, as shown in FIG. 10, an image exposure is applied by an imagewise light energy 61, whereby the photosensitive particles in the exposed portions generate positive and negative charges, the positive charge is transported to the surface of the layer to neutralize the surface charge and, at the same time, the negative charge remains in the particles. Then, as shown in FIG. 11, when the surface of the master plate is heated by heat energy 62, the charged particles in the photosensitive particles migrate in the softened layer by the electrostatic attraction. That is, by heating, an image holding member 90 of the state having the portions containing particles 842 migrated according to the image by heating and the portions containing particles 841 held in situ without migrating is formed.
For carrying out copying using the image holding member 90 in such a state, as shown in FIG. 12, the surface of the image holding member is uniformly charged positively by mean of a corona charging device 71. Then, as shown in FIG. 13, the whole surface is uniformly exposed to light 72, whereby the photosensitive particles generate charges but in this case, since the portions having the particles 841 existing in the original position are different from the portions having the migrated particles 842 in potential decaying characteristics, in other words, since the portions having the particles 841 have a higher sensitivity and decay faster than the portions having the migrated particles 842, there occurs a difference in surface potential between both the portions and an electrostatic latent image of a high electric potential is formed at the portions having the migrated particles 842.
Then, as shown in FIG. 14, the electrostatic latent image is developed with toner particles 91 and further as shown in FIG. 15, the toner image is transferred to a receiving paper 92 using a transfer charging device 93 to form a toner image on the paper 92. Thereafter, as shown in FIG. 16, by strongly applying thereto static eliminating light 94, the potential at the portions having the migrated particles 842 is decayed and the holding member returns to the original state.
By repeating the aforesaid steps of from FIG. 12 to FIG. 16 for the image holding member, the copying operations for obtaining many copies can be carried out by charging and the overall exposure only without need of an additional image exposure step. In this case, since the image holding member used becomes unnecessary after finishing the copying operation, a new photosensitive master plate 80 is prepared again.
Also, in the case of negatively charging the image holding member at uniformly charging it in FIG. 12, when an overall exposure 72 is applied onto the surface, the surface charge at the portions having the particles 842 is decayed and the portions having the particles 841 are developed to give a reversed toner image to the case shown in FIG. 14.
The electrophotographic method using the photosensitive master plate described above is a very preferred method in the case of copying many copies by one image exposure. However, since selenium is used as the photosensitive particles, there is a disadvantage that the spectral sensitivity is dominated by selenium.
FIG. 8 is a graph showing the spectral sensitivity characteristics of photosensitive layer, wherein B is the spectral sensitivity curve of a photosensitive layer using selenium.
Recently, with the progress of an electronic technique, a system of operating imagewise modulated laser light with a rotary reflection mirror as a light source of an image exposure has been widely employed and as the laser light source in this case, a laser diode emitting laser light having a wavelength of 780 nm has been generally used. However, as is clear from FIG. 8, since the photosensitive layer using selenium does not have a sensitivity to 780 nm, the foregoing electrophotographic method using selenium as the photosensitive particles has the fault that images cannot be written by such a laser light. Furthermore, in the foregoing electrophotographic system, there is a problem that at copying, the overall exposure as show in FIG. 13 must be repeatedly applied every one copy and hence even though an addition image exposure is unnecessary in the aforesaid electrophotographic system, the simplification of the steps has been desired.